Conventionally, a frequency of an oscillation circuit is trimmed to a target frequency under particular conditions at shipment and is shipped in a state that the frequency is adjusted. However, characteristics of each of devices constituting the oscillation circuit may fluctuate due to temperature dependence, voltage dependence, aging, external stress and the like, and frequency accuracy of an oscillation frequency of an oscillation circuit output may be deteriorated by being influenced by the fluctuation.
Therefore, conventionally, when high-accuracy oscillation output is required, calibration is performed by counting oscillation circuit clocks with a low-speed oscillation clock, an external pulse such as a communication pulse, or the like used as a reference clock. By using such a reference clock for calibration, it is possible to obtain high frequency accuracy. However, there may be a case where the reference clock for calibration cannot be used.
Therefore, an apparatus which performs frequency adjustment without using the reference clock for calibration is examined. For example, a method is conceivable in which an oscillation frequency is adjusted by generating a pulse with a sufficient width, counting the number of output clocks of an oscillation circuit during a period of occurrence of the pulse and estimating the oscillation frequency based on the pulse width and a count value.
Especially in a case of configuring an oscillation circuit on a semiconductor apparatus, it is possible to construct a system which is advantageous for integration by controlling an oscillation frequency without using a reference clock by a crystal vibrator or the like.
In conventional methods for adjusting an oscillation frequency, however, there is a problem that power consumption relatively increases, and it is difficult to secure accuracy due to a factor of fluctuation in each characteristic so that it is not possible to perform high-accuracy oscillation frequency control.